It depends entirely on where the thing is going to be used. As in how long of a run will the coax be. For a mobile, with fairly short runs, any coax that will handle the power output will work just fine, ~IF~ the rest of the antenna system is done correctly. As in is the antenna fairly close to having a 50 ohm input? Having slightly more power handling ability ( 2X ) comes in handy, but there are limits. In most cases, RG-8U will work just fine. In some cases, RG-58U will work too. [Get ready! That statement will bring them out of the woodworks - lol. Still true though.] Then again, 6" hardline would work too... if it'll fit, if you can afford it, and if you're totally nuts! That's one of those "Yabutt" thingys. 'Yabutt', how much difference does it really make on the 'other' end?
- 'Doc
PS - Answer to that last question is none.